(1) Field of the Invention:
This invention relates to a fused silica envelope for a discharge lamp, and more specifically to a fused silica envelope suitable for use in providing an ozone-free discharge lamp.
(2) Description of the Prior Art:
Many of discharge lamps led by xenon lamps and mercury vapor lamps radiate light which contains an ultraviolet ray having a wavelength of 200 nm or shorter. An ultraviolet ray having a wavelength of 200 nm or shorter causes oxygen, which is contained in air, to react so that ozone is formed. Ozone is harmful to human bodies. Accordingly, its formation has to be avoided except that ozone is positively utilized for cleaning, sterilization or the like. Namely, it is necessary to prevent an ultraviolet ray having a wavelength of 200 nm or shorter from being given off from a discharge lamp. For this purpose, the envelopes of discharge lamps are formed with a so-called ozone-free fused silica glass which does not permit transmission of ultraviolet rays having wavelengths of 200 nm and shorter. A fused silica glass doped with TiO.sub.2 and containing Ti atoms dispersed therein may, for example, be used as the ozone-free fused silica glass.
If an envelope made of a fused silica glass, in which Ti atoms are not evenly dispersed and the diffusion of the Ti atoms are hence non-uniform, is used for the fabrication of xenon lamp by way of example, tensile stresses are caused to occur in an inner surface layer of the wall of the envelope by ultraviolet rays radiated from the discharge arc. As a result, cracks occur in the envelope in a short period of time so that the service life of the xenon lamp is reduced. In some cases, strain was developed as early as several hours of lighting, resulting in fracture of the envelopes after lit for 100 hours or so.
The above problem does not arise if us is made on an envelope made of a high-quality, ozone-free, fused silica glass with Ti atoms dispersed uniformly therein. It is however necessary to melt TiO.sub.2 and SiO.sub.2 at an elevated temperature for many hours in order to produce a fused silica glass with Ti atoms dispersed uniformly. Its production is therefore very time-consuming and cumbersome and its price is accordingly high.